Some electrochemical cells are housed in an open ended container having a flexible metal cover hermetically sealed to the open end. A terminal of the cell extends through the cover and is hermetically sealed to the cover by a glass or ceramic bead which is bonded to both the terminal and the cover.
It is important to maintain the integrity of the seal in order to keep the cell leakproof. The glass or ceramic is fragile and easily broken, especially when the terminal is subjected to an appreciable external force as may occur during the manufacturing of the cell or if it is dropped during handling or use. Since glass and ceramic are stronger in compression, the strength of the seal is improved by maintaining it under compression by the surrounding portion of the metal cover.
While the seal must resist fracture by an externally applied force, it is desirable that the seal be frangible in response to a build up of pressure within the cell in order to safely release such internal pressure. Failure to relieve the internal pressure could result in the disassembly of the component parts of the cell. This is particularly true in cell systems employing lithium, high vapor pressure organic solvents or inorganic cathode/solvent materials. These materials are highly reactive and corrosive and must be hermetically sealed in order to prevent exposure to the outside environment and to provide superior shelf life. Typical cell systems employing such highly reactive and corrosive materials include cell systems of lithium/oxyhalide, lithium/MnO.sub.2, lithium/polycarbonfluoride, lithium/TiS.sub.2 and lithium/SO.sub.2.
It is desired that the hermetic seal open at a predetermined internal pressure in order to avoid the aforementioned possibility of the disassembly of the cell components. Moreover, it is desirable that the pressure range between the pressure required to provide a slight venting of the cell and the pressure required for a blow out of the seal be within a relatively narrow range. This will allow for removal of the seal and provide a relatively large vent opening should the pressure continue to rise after the opening of a small vent.
U.S. Pat. No. 4,115,629 discloses a cell closure comprising a flexible metal member having a vent opening of predetermined area spaced from the cell terminal. Over this opening a metallic disc of a size larger than the opening is symmetrically positioned and a glass or ceramic layer is hermetically bonded to the closure and to the disc. The structure of the bond is such that a predetermined gas build up within the cell will cause the vent to rupture thereby venting the gas from the cell. Rupture occurs when the internal pressure rise causes the closure to flex outwardly to the point where the effect of the flexure becomes great enough to produce a shear force sufficiently large to fracture the glass or the bond between the glass and the metal.
In U.S. Pat. Nos. 4,047,292 and 4,233,372 the metallic cover of the cell is provided with an opening and there is a flange about this opening which extends inwardly of the cell container. In the '372 patent, the flange extends generally parallel to the side wall of the container whereas in the '292 patent, the flange is flared so that the cross-sectional area of the opening on the inside of the container is larger than the cross sectional area at the exterior of the container.
European Patent Publication No. 61,725 discloses a hermetic seal for a cell including a flexible cover having a peripheral skirt sealed to the open end of the cell container and an inner disposed flange defining an opening through which protrudes the terminal of the cell. The terminal is hermetically sealed to the flange using a layer of glass or ceramic bonded to both the terminal and the flange. The flange in this European application extends substantially parallel to the wall of the container wherein flexing of the cover causes the hermetic seal to fail and break.